This article said 17.
“The sale included 17 acres of land and all the original home furnishings. He also later purchased the beachfront property in front of the estate.”
https://www.cheatsheet.com/entertainment/donald-trump-mar-a-lago.html/
The later purchase of the beachfront may have made it the 20-24 acres.
Re: Mar-A-LagQ
I like your numbers better